You can't date my sister!
by Gleek-Whovianfangirl
Summary: Marauders Era James has a twin sister named Ellie, what happens when she turns out to be the next girl on Sirius' list. James has to convince his sister to not talk to him while at the same time try get Lily to go on a date with him (I suck at summaries! But cookies for anyone who reads) Lily/James Sirius/OC
1. Chapter 1

Ellie Potter - Description/bio

Ellie is James' twin brother, not identical though. She also happens to be best friends with the one and only Lily Evans. She has long, brown hair that reaches the small of her back just about. She also has light hazel eyes that she wears muggle make up with just to help them stand out more. She is close to all the Marauders, including maybe a little bit flirty with Remus and Sirius but it's all for fun. Ellie is about 5'6 and enjoys Quidditch, even though she's not on the team, reading and annoying her older brother.

Disclaimer: I own nothing but my OC character. All rights go to the queen J.K Rowling.

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Chapter 1

It was that time again, September the first and this time they were all heading back to school as sixth years, considering Sirius had pretty much moved into the Potters house the three arrived at platform 9 3/4 together, stopping just outside the train carriages, the boys looking for Remus and Peter while Ellie stood on her toes, looking over the top of other wizards heads for Lily. Lily refused to stay over for a few nights because of James. He'd pestered her all of fifth year and she just wanted a break. The minute Ellie saw Lily's fiery red hair she dropped her trunk on Sirius's foot, she ignored the yelp from her friend as she dodged through the crowd and jumped on her best friend, pulling her in for a big hug.

James glanced over at his best friend, laughing hard when he saw the pure agony in his face. "How much junk do girls need to take?" Black as James, pushing the heavy trunk away from him, leaning on his friend. "Where did she go anyway?" He frowned, trying to spot her. James on the other hand just shook his head, he could see them clearly, he must admit Lily is a whole lot prettier than last year. James smirked to himself, this year he _was_ going to get at least one date out of her, he had to. Sirius on the other hand knew his friend way too well. "She's with Lily right? You should stop drooling Prongs, that's just a total turn off." Sirius told him, patting his shoulder as the two girls came back over to them.

Ellie understood her brother could be a jerk to Lily, so instead she planned on grabbing her trunk and just walking to find a compartment on the train they could share. She quickly hugged her brother, then Sirius, whispering 'Sorry' quickly in his ear. She quickly pecked his cheek in nothing more than a friendly manor and back up a little. James didn't fail to notice the small action and frowned, deciding to bring it up l later. "Hey Evans, how about a hug for me?" He teased holding his arms apart for her. Lily just shook her head with a frown and turned around, flipping her hair in the process.

"Nice try Jamsie, just don't be such an idiot this year okay?" Ellie smiled before following her friend into the train, getting help from a strong 7th year as her trunk got lifted into the train. She gave him a cheeky wink before taking Lily's hand and pulled her into a nearby empty compartment. "Sorry about James..again..He's not grown out of being a bloody prat yet." She apologised, taking the seat at the window as the ginger sat opposite her.

"We still have the agreement I can hex him if he gets too close or does anything stupid right?" Lily asked while she twirled her wand in her fingers. The brunette nodded with a grin. "Just make sure you have some method of recording it, that way I can watch him get humiliated over and over again." She grinned as she noticed the train pulling away from the station. "I'm going to get changed if that's alright with you. That way I can get comfy for the ride." She picked up her bag and left the compartment, not noticing James slip in straight after and Sirius follow her, well that was until she felt a tap on her shoulder.

**Lily's POV**

Lily heard the compartment door slide shut and just assumed Ellie had forgotten something. "You forgot your tie again didn't you?" She turned around and just groaned when she saw the smirking figure of James leaned against the now shut door. "Can you please just leave me alone?" She requested and leaned back into the padded seats. "I don't like you so you're wasting a lot of time."

James pretended not to be a teeny bit hurt by her comment and just sat down. "Who says I'm not waiting for my little sister to come back?" James asked, he wasn't waiting for her, he'd seen enough of her at home, but he just played it off. "I think Black's stalking her too." Lily raised an eyebrow, Sirius wasn't the best man to leave around pretty girls he was yet to kiss.

"Is that a good thing? I mean it's Sirius." She paused, she made a promise to herself that if he did the one liner of 'Yes, it is serious.' she was going to slap him so hard. She just got a shrug and he was quiet for another minute.

"So Lils, going to take this opportunity while we're alone. Do you want to go out with me when the Hogsmeade weekend trips start up again?" Lily took in a deep breath, slowly counting to 5 in her head silently.

**Ellie's POV**

Ellie turned around, her robes clutched against her chest. "Sirius what are you doing?" She asked with a frown as she saw the scruffy haired boy stood behind her. "I thought you'd be filling carriages with stink bombs or something."

"You thing so little of me female Potter." He winked, her nickname wasn't new, it just felt wrong calling her Potter considering that's what he called James.

"I only said it because you did it on the way to and from school last year." She smirked defensively, folding her arms over her clothes. Sirius did his usual smirk back, Ellie couldn't hide the fact that she thought he was good looking, pretty much every girl thought it, the difference was she just kept quiet due to the whole 'bro code' his brother had warned him about when they first met. "Now if you don't mind I'm going to get changed into my robes."

"Don't you need your tie to finish the look?" He asked and waved it in front of her face, it'd fallen out of her grip earlier, hence why he followed her in the first place.

"Give me it back Padfoot." She frowned and reached for it, he simply pulled it back out of her reach, she didn't realise until then they were quite close to each other in the small carriage, apparently neither did he. She bit her lip and blushed slightly and grabbed her tie, quickly walking off to the toilets before he could catch up.

"You're quite cute in that shade of red." He called, a grin on his face.

She totally wanted him.

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**A/N **

**So this is the end of Chapter 1. This is also my first Harry Potter fanfic so reviews, follows and favourites will encourage me to continue!**


	2. Chapter 2

Ellie Potter - Description/bio

Ellie is James' twin sister not identical though. She also happens to be best friends with the one and only Lily Evans. She has long, brown hair that reaches the small of her back just about. She also has light hazel eyes that she wears muggle make up with just to help them stand out more. She is close to all the Marauders, including maybe a little bit flirty with Remus and Sirius but it's all for fun. Ellie is about 5'6 and enjoys Quidditch, even though she's not on the team, reading and annoying her older brother.

I just realised I put twin Brother on the other one, I'm not going to change it but just ignore it :)

Disclaimer: I own nothing but my OC character. All rights go to the queen J.K Rowling.

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Chapter 2

The train ride had gone quite smooth and quick, minus the love/hate relationship going on between James and Lily or the quick glances and flirty chat between Sirius and Ellie but no one really registered what was happening once Remus and Peter joined in, shuffling everyone along the small sofa chairs. They'd arrived at the station and already arrived at the school. They had a short time to settle back into their school and place their smaller bags in the room before making their way down to the great hall.

"Do we really have to sit through all the first years getting sorted. It's boring and stupid." Sirius paused. "Although, spit balls will always be a highlight of the first half of this, obviously the mass amount of food is my most favourite part." He grinned towards Ellie and James, ignoring Lily's quiet tuts of disappointment.

"One of these days McGonagall or Dumbledore will see you and you'll get more detentions than you already have." She muttered and ignored her best friends shove.

"Lighten up a little Lils, He's good at it and he won't get found out." She said confidently before jogging to catch up with Sirius, leaving James and Lily to entertain themselves on the short walk down.

**Sirius' POV**

Sirius walked ahead of his three friends, he wasn't going to get talked out of his usual 'annoying the first years' tradition. He was snapped out of his thoughts when he got tapped on his shoulder, then feeling a hand straighten back out his collar. "You seriously can't dress yourself yet?" The familiar voice of Ellie said close to his ear as they stopped at the doors while students poured into the room. "After seventeen, nearly eighteen it's just expected of us." She smirked and stood beside him now.

Sirius smirked. "Well my clothes are never on me in the company of girls, it's sort of a rush to put them back on." He told her in a whisper.  
Ellie felt a weird sense of jealousy at him mentioning a girl for the first time. "You've already gotten some?" She asked, trying to be surprised. "That's got to be a record, last time it was just after dinner." She smiled. "I'm impressed Black." She nodded as she walked through the huge doors, towards the Gryffindor table.

A while later Ellie was sat between Lily and Sirius and James was sat beside Remus and Peter. The first years had been sorted, spitballs had been thrown in the unsuspecting ones and yet again, he hadn't got caught, however he swore he saw a small glint of humor in Dumbledore's eye. He so knew.

"..And let the feast begin!" Dumbledore said, waking some of the seventh years as huge amounts of food appeared on the table.

Sirius' eyes lit up as a whole turkey appeared in front of him, without a second glance her took about a 1/4 of the large animal and placed it on his plate. Remus scoffed at the amount he'd taken. "Some of us would like dinner tonight too!" He frowned and took some food for himself.

Within 2 minutes the four marauders had taken all the food on the table, menus the vegetables. Leaving Lily and Ellie just to look at the empty, silver plates. They quickly shared a glance before Ellie leaned over Sirius, taking half of his meat with a fork and dropped it on her plate. "Thank you for sharing Black. It's an honour to be your best friend over James." She teased with a wink and started eating the turkey before he could take it back, earning a nudge from her friend. James watched the interaction between the two and frowned, but decided to bring it up later.

"So Lily!" James started before he yelped. "Why did you kick me!" He complained, reaching down to rub his shin. "You've got a harder kick now, I wouldn't be surprised if you had steel in the toe of your school shoes." He complained but quickly started eating again.

* * *

Later that evening they were all sat in the common room, taking up the corner by the lit fireplace, much to everyone else's dismay. Lily had joined them after a large disagreement and then being physically dragged by Ellie to the marauders, claiming 'they were fun to hang around with.' Lily thought otherwise.

Lily was sat at the opposite side of the floor from James, who had hogged all the sofa with Sirius, the others just sitting on the floor, looking up to their apparent leaders. Ellie sighed. "This floor has gotten so uncomfy it's not nice." She muttered. She bit her lip in hesitation before jumping up and diving between the two guys sat on the small sofa, making herself half comfy by leaning on serious.

"Seriously! This is my couch." They both groaned at the same time, earning a laugh from Remus considering how alike the pair actually were.

"No it's not!" She protested and wiggled to fit in a little bit better. "It is the schools, well Gryffindors at least, therefore you have to share you tight arses." She muttered and placed her head on Sirius' shoulder, deciding it'd be more comfy than leaning back on the hard backboard they'd got given after James blew it up last time.

James nudged his sister by 'accident' once she saw her and his best friend get a little too close for his liking again but he quickly tore his eyes away from them and looked back at the gorgeous red head sat in front of him. "What are you even doing down here this late Evans? It's nearly ten thirty, surely you need to start obsessing for tomorrows lessons." He snickered.

Lily rolled his eyes and stood up, he did have a point. "I've had enough of you already and it's not even properly been the first day." She groaned. "I'm going to bed anyway, see you later Els." She smiled at her friend before jogging up the stairs.

Ellie kicked James's leg as best as she could. "Stop being an idiot to her or you're never even gonna make it to first base." She hissed, resulting in a half frown, half glare from him. Yet he failed to deny it.

She picked herself up from the sofa once more, deciding she should probably go to bed too, it'd been a fairly long day for them all. She leaned down and hugged her brother like she did every evening before she went to sleep. "Love you." She whispered and pulled away with a smile before rubbing Remus and Sirius' hair as she walked past them. "Night boys!" She grinned before following Lily's steps up the stairs.

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**A/N**

**Drama follows when the lessons start so hold on :D**

**So I hope I didn't disappoint with this chapter, I'm going to try post every weekend (Or there abouts) however I have my exams coming up soon so I may not follow these deadlines for the next few weeks. _Sorry!_**

**BUT! Reviews, Follows and Favourite make me do chapters quicker and better. So you know, maybe just click a few buttons ;)**


	3. Chapter 3

Disclaimer: I clearly don't own Harry Potter or I wouldn't be writing fanfics!

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Chapter 3

"I can't believe McGonagall had the guts to give us a detention for yesterday." Sirius muttered, toying his cereal with the metal spoon while slouched across the Gryffindor table, two of his friends sat opposite him seeing as Peter was still sleeping. "Good Ol' Minerva knows how long we've been doing it for! Why did she even choose this year to punish us." Sirius whined practically as he dropped his spoon heavily into the bowl, causing milk to splash onto his robe, causing more whines. James and Remus just shook their heads, trying hard not to laugh and pretend to feel sympathetic. They'd gotten out of detention so it was just Sirius who had to attend.

Lily slowly walked in, she looked happy considering how early it was. Then again she was excited to start school again. She sat herself down, beside Remus as he was the only one she could properly stand out of the four. She also knew when her best friend decided to wake up properly she'd come and sit with them all. "Hey Remus and Sirius." She smiled, purposely missing James out, which caused a confused look from the messy haired boy. "And Potter." She continued in a mumble, just loud enough for him to hear. She grabbed some toast and butter from the center of the table and placed it on the plate that seemed like it had been waiting for her to arrive.

"Evans." James mumbled back in reply to her, just to be polite. He then continued the earlier conversation with Sirius. "On the plus side, for us anyway. We're not getting told off by her. She clearly just likes us more than you." He teased with a wink.

Sirius scowled at his best friend. "Hey! She loves me really, hence why she wants to spend more time with me in detention." He said, raising his voice slightly to try get his point across to them all. Including Lily who snickered under her breath. "It clearly is!" He continued to try win the argument. "Why do you think it's always her who gives me detention?" He questioned.

"Because she always catches you, while in the middle of pranks. Dumbledore finds them the greatest things ever beside Lemon Sherbets and the rest of the teachers are too dozy or caught up in life to care. Then there's the few who know a detention won't do anything to make you stop what you got it for." Remus pointed out with a wise tone in his sleepy voice. "In other words. She's the only one who properly enforces the rules."

Sirius rolled his eyes, fighting the urge to come up with some clever answer seeing as he knew Remus was right."Sure. Whatever." He muttered and shoved a few more spoonfuls of his already half eaten cereal into his mouth greedily. "I'm going to go grab my robe. I'll see you in class." He told the three of them and quickly got out, fleeing from the main hall.

Remus glanced at the other two. "I'm just gonna go." He smirked towards James and quickly followed Sirius' lead, but then turned into the library deciding to get some last minute studying in. He didn't need to, they weren't going to have tests but he just wanted to leave his friends alone.

* * *

Lily watched Remus leave, a frown on her face as she turned back to face James, his mouth open to start talking. "Don't." She ordered, knowing what he was going to say straight away.

James frowned back at her. "I-I wasn't going to say anything other than your hair is sat in the crumbs from the bread.." He shrugged. It was the truth. He then looked down at his breakfast, starting to finish up with it. Earlier in the morning, he'd decided to stop pestering her and just be nice. At least start off as friends before he asked her out again. He guessed he would have a better shot at her actually saying yes that way.

Lily looked down, quickly sweeping the crumbs away from her red locks. "Oh.." She mumbled, feeling herself blush gently at her accusation. "Sorry." She whispered and pushed the plate away so it wouldn't happen again. "What are you in first?" She asked, trying to make the whole situation a little less awkward.

James snapped his head up to look at her, she as actually talking to him. Not hexing. Not cursing. Not telling him off. Just simple conversation. "Um..Potions with Slughorn I think." He shrugged, a small smile find its way to his face. "How about you?" He asked.

Lily groaned quietly, not sure if he actually heard her. "Potions with Slughorn. Let me guess. The Marauders are in there too?" She asked, hoping at least Sirius wasn't in there. He was loud and could be a mass distraction. She needed to focus even more than the last this year.

James nodded with a smirk, they'd be spending more time together. "Yes they are!" He grinned enthusiastically. "Now, I'm going to head to class. Care to join me? I'll try my hardest not to annoy you." He half promised and stood up, brushing crumbs from his sweater.

Lily nodded, she was going to head that way anyway. "Sure."She nodded, matching his movements and stood up, slowly starting to make her way towards the massive doors at the end of the main hall, James following her at the other side of the table, leaving the room beside her.

James held his arm out to stop Lily from walking any further as they were a few steps away from the classroom, stopping at the last corner. "Shh.." He mumbled, hearing Sirius talk to someone, he was just going to wait to hear the victims voice to judge who it was.

"You were going to tell him." Sirius mumbled to the girl. "You said yesterday you were going to tell him because you knew he guessed something was going on between us." He told her, letting his arms fall either side of her shoulders, to stop her trying to get away like she did earlier in the morning.

"It's not easy telling your brother that you're having sex with his best friend. What if he doesn't even know about _that_ part, he could just think something else." She hissed back at him, leaning back into the wall as much as possible as he got closer, feeling a little intimidated.

James paled and Lily glanced up to him. He'd figured out who it is. He looked shocked and looked down at her with wide eyes as he tried to figure out what was happening. "It can't be her." He mumbled to no one in particular, glancing at the floor. He took a breath before stepping into the hall, he planned on just playing it off, pretending like he didn't hear anything. He wanted to see how long it'd take for her, or him, to own up. "Oh hey guys, didn't think you were one to get here early." Black he teased, trying to sound normal. It was very hard just to look at them.

Sirius just rolled his eyes in response and looked at Ellie, almost urging her to tell him with just a nod. "Your dear sister wants to tell you something." He told James, that famous 'Black' smirk on his face.

Ellie scowled. "No I don't!" Before anymore arguments started Slughorn opened his classroom doors and Ellie walked in as quick as possible, a few random Ravenclaws walking in with her. She didn't look back and took her seat near the front so it'd be harder for them to bother. Lily came and sat beside her but didn't mention anything, just started talking about how excited she was for the day to officially begin with the lessons.

Remus walked in past the two boys who were just glaring at each other, curiously. Sirius took this as his chance and quickly followed Remus in, sitting beside him, leaving James to sit somewhere else where he couldn't interrogate him, Sirius knew it was coming. He just wanted to hold it off for as long as possible.

* * *

**A/N**

**So I thought I'd update sooner seeing as I may be studying for my English exams and I wanted to leave this. So maybe you'll have another one, but if not Enjoy!**

**Review, Follow or Favorites would be lovely :D**


	4. Chapter 4

Chapter 4

As soon as they were out of potions James launched himself at his friend, the friend that had gone behind his back and slept with his sister. "What the hell!" James said loudly as he pinned Sirius to the cool wall by his cloak. "Of all the people you chose her. Why!" He didn't really give Black any time to answer as he held him tighter against the wall, grabbing his wand from his pocket and jabbing it into his arm. "Don't think I won't." He threatened him. That was before the wand went flying out of James' hand and into Lily's, Ellie stood beside her, arms folded across her chest.

"Put your best friend down, now, before I hex you." She warned and stepped closer. She wanted to take blame for putting her brother and their friend against each other. "It's my fault as much as his, so I'm going to ask you kindly. Don't kill Sirius." She said and put her hand on his arm. "We can all talk about this later, when we aren't surrounded by other people."

As if being he was being controlled, James let go of him, letting Sirius slouch against the wall while James took a step back. "I'm not done with either of you." He warned the pair and walked off, the halls parting for him like the red sea, sensing the fact he'd seriously harm anyone in the way of him until he was completely calmed down.

Ellie looked over at Sirius, before slowly walking over to him. "Come on." She muttered as she took his arm, pulling him along with her gently until they were out of the crowd. "So much for he knew." She mumbled under her breath to him, word would get around pretty quickly. It was Hogwarts. She just wanted that last little bit of privacy. "If you'd have kept quiet, I could have let him on easily." She told him with a sigh as she looked at him. "You're quiet, you're not hurt are you?" She asked, pointing out his strange...sane personality.

He shrugged. "I'm fine. I should have kept quiet, I know I'm gonna get a beating back in the common room later. I also take it, whatever this was, is now over." He said, a fake smile on his face as he looked up at her; she just showed signs of thought.

"Lets just say it's on hold while we sort things out with my dear brother." She teased nudging him gently, bringing a smirk to his face, before she walked ahead. "After dinner just don't try talk to James on your own, you'll probably end up dead." She told him honestly. "Let me take some of the blame." She told him, before walking into the library to find Lily before their next lesson.

Ellie quickly found her friend, slotting herself into the seat beside, ignoring the judgmental glare she was receiving. "Go on, say what's on your mind, I won't be offended." Ellie sighed, knowing her friend too well.

"Him! Really!" She almost cried, quickly getting shushed by the librarian. She continued but in a quieter voice "You always go on about how great he is, but you and I both know he's pretty much the schools biggest player." She sighed. "I just don't want to see you get hurt."

The brunette just shook her head "I won't, but if I end up doing so, I'll have had it coming and you can then say 'I told you so' " She teased and took a book from her bag to study from. "But, maybe, being a great friend you could take James mind of off it by maybe going on a date with him." She smiled hopefully.

"Absolutely not!" Lily frowned, her response quick. "I'm not getting you out of your troubles only for mine to start with your brother. Once you're involved with him there's not much chance of getting out. No offence." She got a shrug in return.

"Please.." She begged. "I know you don't hate him _too_ much to be there for a night. I can be the one to break it to him that I told you to. I'll even buy you as many chocolate frogs as you want when we go to Hogsmeade" She pleaded, her eyes looking up as she tried to convince her. "Best friends help each other out in times of need." She tried still, Lily was stubborn and wouldn't give up.

"Well it seems like Sirius has been helping you out when you want it. Go to him." She snapped as she stood up, she shouldn't be mad, it wasn't really her battle to fight with but she felt betrayed. Betrayed in the way her best friend didn't tell her what was going on when she was leaving the dorm at stupid o clock every morning. "I'll see you later." She hissed as Madame Pince, the librarian, was doing her rounds of the library and with that she left.

* * *

As Lily rushed out she instantly regretted what she did, to her friend none the less. As she ran out she bumped into someone, however she knew exactly who it was without needing to look..

"James!" She gasped, he was really wearing her down and she gave up being rude, unless necessary to him.

James' arms went around her to help her keep her balance but he quickly let go when he knew she was okay, not needing a slap.

"I need to ask you something," She smiled as she starting talking and couldn't find the will to stop. She owed this to Ellie. "Can we sneak out of Hogwarts tonight..and maybe go on a date?" She winced noticeably at the words she vowed never to speak to James. He just frowned.

"Ugh..Yeah..If you want to." He nodded, a smile covering his face instead. "I'll meet you in the common room later." He told her and walked the opposite way to the library.

Lily let out a breath of relief as he did, she had saved her friend for the night. This would only be a one time thing with James. Nothing more would happen. Or so she hoped, recently James had this _thing _about him she couldn't quite put her finger on.

* * *

**A/N**

**So I haven't updated in ages, got caught up with all my exams but I am now off of school for the summer so *Fingers crossed* I should be able to update more regularly. In the next chapter: Lily and James' date and more Sirius/Ellie drama! **


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